The invention relates to reflective sheeting of the type in which a light-returning layer of contiguous, small, light-reflecting beads refracts and reflects a beam or ray of incident light back toward its original source. The beads act as spherical lenses. Such sheeting is sometimes termed "reflex" or "retroreflective" to distinguish from mirrors which cause specular reflection and from diffusing surfaces which dissipate the incident light in all directions without selective return in a direction substantially parallel to the original direction of incidence. Reflective sheeting of the type described is, accordingly, able to reflect or return incident light at far greater distances than would be possible if diffused reflection occurred, and it does so without sacrifice of other desired performance characteristics. The sheeting may be cut to size and used wherever reflex or retroreflection is desired, such as for various signs and markers, for example, road signs, advertising, displays, and the like.
In the past, light-reflecting beads have been arranged in reflective sheeting in the same plane, such as is shown in U.S. Pat. No. 2,354,049 to Palmquist and in U.S. Pat. No. 2,326,634 to Gebhard et al. According to these patents, beads are partially embedded in a bonding layer which extends without interruption over a planar backing layer, so that the beads themselves are oriented in coplanar fashion, their bottom portions sitting in the bonding layer.
However, beaded surfaces which are not covered in some fashion can have their effectiveness as retroreflectors materially decreased when mud, dust, dirt, and the like eventually deposit on them. Since highway signs cannot be continually wiped clear because of maintenance costs, it becomes desirable to have a retroreflecting surface which is efficient and at the same time relatively smooth. Such a smooth surface is less conductive to dirt and dust buildup and permits rain to wash off any harmful accumulation that may occur from the retroreflecting surface.
Reflective sheeting of this type is described in U.S. Pat. No. 2,948,191 to Hodgson et al. In this case light-reflecting beads are contacted on one side by a layer of air. In particular, a thin, transparent covering film is stretched over the exposed surface, and means are provided for holding the film in a fixed position with respect to the beads and a backing material. The covering film may be attached at certain anchoring points which rise above the level of the light-reflecting beads in such a manner as to leave an air layer between the light-reflecting beads and the smooth surface created by the transparent film. The anchoring points may be supplied by spaced beads having diameters greater than the remaining light-reflective beads. Or the anchoring points may be supplied by a mesh or grid work which raises the covering film sufficiently above the beads to provide the air layer. The presence of a layer of air above the light-reflecting beads permits the use of beads at their maximum efficiency and under optimum conditions. At the same time the film used to maintain the air layer above the beads furnishes a smooth surface which remains relatively free of dirt and which is more effectively washed clean by rain and the like than if the beads were left uncovered.
A very similar arrangement is disclosed by U.S. Pat. No. 3,190,178 to McKenzie which discloses a sheeting having a layer of lens elements exposed to an interface of air and containing a plurality of hermetically isolated groups of such lens elements. McKenzie forms an intersecting grid pattern of pressure-formed, heat-sealed connections between a transparent cover film overlying the beads of the sheet material and the structure of the sheet material underlying the beads.
In such cases of a reflective sheeting having a covering or face film overlying units of light-reflecting beads separated from the film by a layer of air, the practice has been the same as for reflective sheeting of uncovered beads; namely, the beads are placed in a side-by-side, substantially coplanar relation.